Group II-VI compound semiconductors such as cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) are known as semiconductor materials for forming substrates of devices for detecting radiation or infrared rays. Crystals of CdTe and CdZnTe (hereafter “Cd(Zn)Te”) containing no impurity (undoped) or containing a group 1A element, such as sodium (Na) or lithium (Li), show p-type electric properties. In other words, a p-type Cd(Zn)Te crystal can be prepared by adding an appropriate amount of impurity, such as Na or Li, to the crystal.
A trace amount of Na or Li, which is contained in the quartz component of a crucible used for producing crystals, intrudes a Cd(Zn)Te crystal during its production process without intentions of manufacturers. The concentrations of these impurities, however, must be reduced in production of the Cd(Zn)Te crystal for a radiation detecting device. Thus, the solidification rate of Cd(Zn)Te has been limited for growing a Cd(Zn)Te crystal, in other words, its crystal growth has been finished before the melt has completely solidified, so as to leave Na and Li in the melt (see Patent document 1).